What made Finch (Michael Emerson) pick John Reese (Jim Caviezel) to be his partner in Person of Interest? In the early seasons, the show focused on the duo of Reese and Finch, who worked tirelessly to solve crimes before they happened. With Finch's hacking and computer skills, and Reese's experience in the military and the CIA, the two made a great team and were able to save a lot of people.
It all started when Finch created an artificial intelligence - nicknamed the Machine - and designed it to predict violent crimes for the government. The original objective was for it to pinpoint acts of terror, but it worked so well that it identified domestic acts of violence as well. Unfortunately, the government had no interest in preventing every murder, so this information was ignored and discarded. After faking his death, Finch decided to use the Machine to help these people himself. The problem was that Finch only had access to one number and he didn't have the skill set to do what needed to be done alone. That's why he had to recruit someone else to help him. After going through a few other partners, Finch recruited Reese in the pilot episode.
Reese was the perfect candidate for Finch's partner. Like Finch, Reese was believed to be dead and was someone that no one was looking for, but that's not the only reason why Finch set his sights on Reese. A flashback in Person of Interest season 3 shed some light on what drew Finch toward Reese. One year before season 1, back when Reese was still with the CIA, Reese and his partner Kara Stanton (Annie Parisse) were on the trail of a man named Casey (Joseph Mazello), who had worked on the Machine. His knowledge of it made him dangerous, so his death was ordered by the government. Reese was led to believe that Casey was a traitor, and after realizing the truth, he opted to let Casey go and lied to Stanton by telling her Casey was dead. Unbeknownst to Reese, he was being observed by Finch.
What Reese did clearly left an impression on Finch. And what Finch saw was a man who was willing to put human life above his duties to the government. So when Finch discovered later that Reese was still alive, he knew that Reese was the kind of person he had been looking for. The first few episodes of Person of Interest season 1 proved that Finch had made the right choice. He was troubled at first with some of Reese's actions, including his association with Fusco (Kevin Chapman), who was a dirty cop at the time. Even so, it didn't take long at all for Reese and Finch to develop a strong bond. This bond became a central aspect of the show that lasted until the end of the series.
The strength of their friendship and the loyalty that Reese had for Finch was due in large part to Finch's decision to recruit Reese in the first place. Reese, who had nothing when Person of Interest started, always felt indebted to Finch for giving him a purpose. This was reflected in the series finale when Reese gave up his life to ensure that Finch - the person who gave him a second chance - survived the final battle with Samaritan.